I had the same issue with my iPhone6 ... Warranty was going to expire in a week so I visited the Apple Store. They replaced the screen but the issue was not solved. So they just replaced the phone for me. Unfortunately this new one is having the same issue. This is the second iPhone6 replaced by Apple for me and also the display on the old one was replaced twice including this one. I am assuming this is an issue with the iPhone6 model in general. Mine is not very noticeable during normal use but during the night when everything is dark in the room I can see it on my phone at minimum brightness. I still have 3 days of AppleCare+ left. But I think I should keep this one since the next one might have the same issue and there probably wont be a solution for it.